Conventionally, a sheath heater in which a heating coil embedded in insulating powder is placed in a bottomed cylindrical metal sheath is used for starting a diesel engine. In such a sheath heater, however, the thermal conductivity is low, and a long time is required for raising the temperature because the heating coil is embedded in the insulating powder. Recently, it is requested to raise the heating temperature of the heater to 1,000° C. or more, and the time period of afterglow in which the heater generates heats even after the engine is started tends to be prolonged. For the requests, a sheath heater has a problem in durability because the heating coil is made a metal.
Therefore, a ceramic heater has been developed in which a heating portion essentially comprising: a conductive ceramic material such as molybdenum carbide or molybdenum silicide; and an insulative ceramic component such as silicon nitride is embedded in a support made from silicon nitride ceramic that has highly corrosion resistant at a high temperature, whereby the thermal conductivity is improved and a rapid temperature rise is enabled.
An example in which, in such a ceramic heater, a lead portion to be connected to an internal heating portion is configured only by a metal wire such as tungsten (W), and that in which such a lead portion is configured by both a low-resistance ceramic material and a metal wire are disclosed (for example, see Patent References 1 and 2).
Patent Reference 1: JP-A-4-268112
Patent Reference 2: JP-A-2002-334768